Scanning devices such as flat-bed color scanners and color copying machines operate by capturing light reflected from a scanned medium and using the captured light to reproduce images, graphics and/or text from the medium. In a typical scanning device, a light source assembly projects a thin, linear light pattern, that is reflected and then captured by a photo-sensing component. A common photo-sensing component of a scanning device is a charge-coupled device (CCD) having an array of photosensors positioned in a thin strip. The light source assembly is positioned adjacent to the CCD in order to effectuate the projection and capture of the linear light pattern. Depending upon the characteristics of the light being received by the CCD, the definition and color composition of the images, graphics and/or text on the medium can be determined. An entire surface of the medium can be scanned by moving the light source assembly and the CCD relative to the medium. Alternatively, the medium may be moved relative to the light source assembly and the CCD to provide scanning of the surface of the medium.
There are various types of conventional light source assemblies for scanning devices. A conventional light source assembly may include a fluorescent tube having a longitudinally extending aperture formed by removing one or more interior coatings along a lengthwise region of the bulb. A linear illumination is provided by the generated light that escapes from the fluorescent tube through the aperture. However, a more common type of light source assembly for scanning devices utilizes a transparent solid rod to receive light from an electric lamp and to transmit the received light in a direction perpendicular to the length of the rod and along the entire length of the rod to provide the linear illumination. These rod-type light source assemblies tend to be less bulky and require less power than the tube type light source assemblies.
U.S. Pat. No. 4,924,357 to Yamashita et al. discloses a light source assembly that utilizes the above-described rod for business machines, such as scanners and copying machines. The light source assembly of Yamashita et al. includes a lamp that is positioned at one end of a solid transparent rod to emit source light through the interior region of the rod in a lengthwise direction. Various lamps are disclosed that may be utilized in the light source assembly of Yamashita et al. A tungsten halogen lamp, a mercury lamp, a xenon lamp and a flash lamp are specifically identified as possible candidates for generating the light energy for the light source assembly. The rod is described as a solid light-transmitting rod having a circular or elliptical cross-section. Along the length of the rod, a strip of diffusion material is coated onto the outer surface of the rod. The entire rod is enclosed in an aluminum cylinder encasing, which has a slit along its length. The rod is positioned in the encasing such that the diffusion strip on the rod is located opposite to the slit. The diffusion strip operates to diffuse light from the lamp that is propagating through the interior region of the rod. In effect, the diffusion strip redirects the source light from the lamp, that is being reflected from the surface of the rod and the interior surface of the encasing, towards the slit in the encasing such that the redirected light can emanate from the entire length of the slit. The emission of light from the slit in the encasing provides a linear illumination.
A still more power-efficient light source assembly is described in U.S. Pat. No. 5,753,906 to Gennetten. The light source assembly of Gennetten utilizes light emitting diodes (LEDs) to provide the linear illumination. LEDs have a number of advantages over electrical lamps. For example, LEDs are less expensive and more compact than electrical lamps. In addition, LEDs operate at a significantly lower voltage that electrical lamps, while providing a stable geometrical light output, i.e., uniform illumination. The Gennetten light source assembly includes four different types of LEDs, wherein each type of LED is designed to emit light of a particular color. In one embodiment, the LEDs are positioned along a straight line in an alternating fashion. The LEDs are placed in groups of four LEDs, such that the same color LED will be present every fourth LED. In another embodiment, the LEDs form a 4.times.N array such that a row of N LEDs in the array is of one color. In either embodiment, the linearly distributed LEDs provide the linear illumination required for scanning devices.
Although the Gennetten light source assembly utilizes LEDs which have advantages over conventional lamps, the great number of LEDs that are required by the Gennetten light source assembly counterbalances some of the advantages. The Gennetten light source assembly may require tens of LEDs, whereas the Yamashita et al. light source assembly only requires a single lamp.
What is needed is a compact light source assembly for scanning devices that exploits some of the advantages of rod-type light source assemblies, while also taking advantage of the favorable properties of LEDs.